From 2792752291592773988 X-Google-Thread: f78e5,ff39f62da1e60460 X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news3.google.com!news.glorb.com!newsfeed.stueberl.de!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: jpotter@lhup.edu (John Potter) Newsgroups: comp.std.c++ Subject: Re: Temporary object's lifetime and Full-expression Date: Sat, 23 Apr 2005 01:21:22 GMT Organization: EarthLink Inc. -- http://www.EarthLink.net Lines: 124 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: news.demon.co.uk 1114219288 23022 158.152.254.254 (23 Apr 2005 01:21:28 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Sat, 23 Apr 2005 01:21:28 +0000 (UTC) X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Virus-Scanned: by amavisd-new at cs.mu.OZ.AU X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id j3N1LMlC007272; Sat, 23 Apr 2005 11:21:22 +1000 (EST) X-NNTP-Posting-Date: Fri, 22 Apr 2005 17:52:35 PDT X-Delivered-To: std-c++@ucar.edu X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Newsreader: Forte Free Agent 1.93/32.576 English (American) X-Newsgroups: comp.std.c++ Xref: g2news1.google.com comp.std.c++:4575 On Fri, 22 Apr 2005 19:44:48 GMT, stkim@yujinrobot.com ("Kim, Seungtai") wrote: > "What about this case? Is it legel and dose it produce well-defined behavior?" > struct A { > A& me() { return *this; } > }; > void foo(const A* a); > ... > foo( &A().me() ); > I said it's not. It will make the undefined behavior. The reason is that > the temporary object will be destroyed at the point of immediately after > calling function foo. Therefore foo will get the dangling pointer. It's > illegal use of invalid pointer. No. > But the others assist the opposite opinion that the temporary object > will not be destroyed until the completion of the full expression > calling foo. Yes. The major problem in all that follows is quoting out of context. > My opinion comes from the definition of the temporary object's lifetime > and the full-expression. 12.2/p3 states the general rule for lifetime of > the temporary object. > 12.2/p3 > Temporary objects are destroyed as the last step in evaluating > the full-expression (1.9) that (lexically) contains the point > where they were created. Fine. > And 1.9/p12 states the definition of the full-expression. > > 1.9/p12 > A full-expression is an expression that is not a subexpression > of another expression. Fine. > And 1.9/p13 describes the function call is full expression, also. > 1.9/p13 > ... the function call is a full-expression. No, you are out of context. The context is initialization which contains no syntactic expression yet is treated as a full-expression consisting of a function call. struct S { S (int); }; void f () { S s1(1); // No expression, but treated as function call S s2 = 2; // Same } The text does not say that all function calls are full-expressions. It says that these two cases are full-expression function calls. > In this point, there can be two different but ciritical interpretations > whether the function call in an expression is a sub-expression > or not it's an independent full expression. Not if you read everything and not just part. > - The function call is sub-expression. It's explicitly included in > the other expression. So, the temporary object should not be destroyed > until after evaluating full expression that contains the function call. > Therefore OP's above code is legal and produce well-defined behavior. Yes. > - The function call is full expression that it is independent > from the expression containing the function call. For that reason, the > temporary object shold be destroyed immediately after calling function. > Hance, above code will make UB. No. > I have the more questions if we assume that the function call is a > sub-expression of the other expression. > 1. Why dose the bellow sentance exist? > > 12.4/p5 This is 12.2/5. > ... A temporary bound to a reference parameter in a function > call(5.2.2) persists until the completion of the full > expression containing the call. ... You are again missing the major point. A temporary bound to a reference normally has the its lifetime extended to that of the reference. This is an exception to that rule because the temporary outlives the reference which terminates upon return. > If function call is a sub-expression, it's not required. Always > the temporary object survives until the completion of the full > expression containing the function call. Why it needed? To emphasize that it outlives the reference which is the subject of this paragraph. > 2. Why dose the 1.9/p13 emphasize "function call is a full expression." > twice. See above. It is talking about two special cases where a non-expression syntax is treated as a full-expression. John --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ]